1. Field of the Invention
The present invention relates to a turbocharger used in an internal combustion engine, especially to a method for assembling an impeller onto a turboshaft.
2. Description of the Prior Art
Turbochargers are being increasingly used in internal combustion engines to obtain higher power. Turbochargers, which deliver a supercharge of air to the engine, are used at very high speeds which may exceed 100,000 rpm. Therefore, it is very important for them to be balanced in their rotating mass. The rotating mass of turbochargers comprises the turbine spun by the exhaust gas, the impeller or compressor rotor spun by the turbine through the shaft, and other elements such as spacers, thrust collars, and nuts mounted on the shaft. To obtain a well balanced rotating mass, the above components are machined to a high precision. The turbine and the impeller are especially important due to their functions, sizes, and masses.
These components, however, have to be assembled into a unit and even if each component is precision machined, it is still difficult to obtain a well balanced assembly. It is not so difficult to balance the turbine relative to the shaft, as the turbine is integrated together with the shaft and machined as a unit, but it is very difficult to assemble the impeller onto the shaft. The shaft integrated with the turbine is known as a turboshaft.
Conventionally, the shaft is first inserted into an axial hole in the impeller, than the impeller is clamped by a nut with a force great enough to enable torque to be transmitted from the shaft to the impeller and to fix the axial position of the impeller. The great force by which the nut is clamped in this operation tends to bend the part of the shaft holding the impeller. Further, the shaft may be clamped offset from the impeller center axis within the clearance between the shaft outer wall and the inner surface of the impeller hole.
To prevent the above-mentioned bending and offseting of the shaft, it is desirable to minimize the clearance between the shaft outer wall and the inner wall of the hole. This, however, would reduce the efficiency of assembly work. In practice, also, zero clearance cannot be achieved due to the need for dimensional tolerances even with precision machined components.